Image alignment system for use in laser ablation treatment of the cornea and associated methods

ABSTRACT

A system and method for aligning a first and a second image of an eye includes making a determination of a limbus location on a first eye image and a second eye image. The limbus location of the first and the second eye images are then aligned in two dimensions. A second eye feature location is determined on the first and the second eye image. One of the first and the second eye image is relatively rotated, and a correlation is performed on the first and the second eye image at a plurality of relative rotational positions using the second eye feature location. From the correlation an optimal first and second image alignment is identified.

FIELD OF THE INVENTION

The present invention is directed to laser surgery on the eye, and, moreparticularly, to laser ablation surgery for correcting visualimpairment, and, most particularly, to systems and methods for achievingalignment between images from an analytical device and a laser surgicaldevice.

BACKGROUND OF THE INVENTION

In custom refractive surgery on the eye, the analysis portion of theprocess is typically carried out prior to the surgery, for example, witha wavefront aberrometer. Subsequent to the wavefront measurement, thelaser ablation portion is directed to follow a prescription generated bythe analysis portion. These procedures are often separated by days.

It is desirable to align the aberrometer results with the surgicalsystem so that the ablation is positioned properly. Aligning images ofthe eye with high precision is challenging. The pupil, which can be easyto find and align, varies in size over short intervals. Further, theposition of the pupil within the iris changes depending upon whether itis contracting or dilating, making high-precision alignment of two eyeimages difficult with the use of the pupil.

The limbus remains fixed between two images, but presents a softboundary that changes appearance with illumination. A difficultalignment component is achieving relative rotation between the twoimages, since the main alignment markers, the pupil and the limbus, arerotationally symmetric. It is known in the art to use artificialalignment marks placed on the eye to provide a reference. However, thesemarks have conflicting requirements, in that they be stable and easilyseen for alignment, but easily removed after surgery. It would be highlyinconvenient to retain these marks in place for the period betweenmeasurement and surgery.

Algorithms are known in the art for assisting in image alignment;however, the surgeon must confirm this alignment before proceeding withsurgery. It would be beneficial to provide an image alignment processthat verifies alignment and also permits adjustment of the determinedalignment.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method for aligning afirst and a second image of an eye. The method comprises the step ofdetermining a limbus location on a first eye image and a second eyeimage. The limbus location of the first and the second eye images arethen aligned in two dimensions. A second eye feature is determined onthe first and the second eye image. One of the first and the second eyeimage is relatively rotated, and a correlation is performed on the firstand the second eye image at a plurality of relative rotational positionsusing the second eye feature. From the correlation an optimal first andsecond image alignment is identified.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an exemplary system schematic for the present invention.

FIGS. 2A,2B is a flowchart of an exemplary embodiment of the method ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description of preferred embodiments of the invention will now bepresented with reference to FIGS. 1-2B, with an exemplary embodiment ofa wavefront measurement of an eye and subsequent laser ablation surgerythereon.

A system 10 (FIG. 1) for aligning a first and a second image of an eyeincludes means for collecting the first and the second eye image. In aparticular embodiment of laser ablation surgery, typically a first imageis collected at a first location 11, for example, using a wavefrontaberrometer 12. This first image is input into a processor 13 havingimage processing software 14 resident thereon.

When the surgery is to be conducted, a second image is collected at thesurgery site 15 using a second imaging device 16. The surgeon typicallywishes to view the second image during surgery on a display device 17,and would also wish to ensure that the “live” eye image is aligned withthe first, previously collected, image so that the determinedprescription is aligned properly with the “live” eye. Therefore, thesoftware package 14 of the present invention is adapted to provide thisalignment, and also a visual confirmation of this alignment to thesurgeon.

The software package 14 includes code segments for carrying out themethod 100 steps of the present invention. The method 100 (FIG. 2)includes the steps of determining a limbus location on a first eye image(block 101) and a second eye image and aligning the limbus locations intwo dimensions (block 102), for example, vertically and horizontally,with reference to the calculated center of the limbus.

A second eye feature, for example, one or more blood vessels, is locatedon the first and the second eye images (block 103), for the purpose ofproviding a rotational reference point, since the limbus issubstantially rotationally symmetric. Other second eye features can beused as well, such as natural eye marks and artificial eye marks. Thismethod is believed preferable to aligning on pixel values, since thefirst and the second image are typically collected with differentcameras under different lighting conditions. The blood vessels can bedetected, for example, with the use of a gradient filter (block 104).Preferably the gradient filter is adaptive to the orientation of theblood vessel in order to maximize the accuracy of detection. If morethan one blood vessel is used, each is assigned intensity values thatare proportional to their prominence.

In an alternate embodiment, individual blood vessels that are common inthe first and the second image are identified, and the images arealigned based upon the location and orientation of these blood vessels(block 105).

Once the limbus locations are aligned, the first and the second imageare rotated relative to each other at predetermined intervals (block106). At each point, a correlation is performed using the blood vessellocation separation as an alignment criterion (block 107). From thecorrelation, an optimal first and second image alignment is identified(block 108).

In order that the first and the second images be properly weighted, arelative contrast and a relative brightness can be adjusted to achieve asubstantial match therebetween. In particular, the first and the secondimage can be adjusted to have similar contrast and brightness overall(block 109). Next, an area of importance, for example, the location ofthe blood vessel(s) being used as reference, is determined (block 110).This area is subdivided into a plurality of sectors (block 111), and thecolor and brightness of each sector are equalized (block 112). Thesectors may or may not be overlapping, which affects spatialfiltering/smoothing. This procedure results in same details beingapparent in both images.

The calculated optimal alignment is then displayed to the surgeon (block113), preferably including the blood vessel(s). The calculation andpresentation of such a display can take any of a number of forms. Forexample, pixels of the first and the second image can be interleaved toform a composite image; which is then displayed (block 114). Theinterleaving can take any of a plurality of forms, for example,alternating columns or rows, with different dimensions if desired, orpixels arranged in a “checkerboard” pattern with any size squares.Another method uses a transparency feature, wherein pixels from thefirst and the second image are displayed using a weighted average (block115). In such a display, the blood vessels can be superimposed on theircorresponding locations in varying levels of intensity based upon apredetermined “significance.”

In order that the surgeon may verify that the calculated alignment isindeed optimal, the invention includes a feature wherein an animation isdisplayed of a relative rotational motion of the first and the secondimage about the optimal alignment. Rotation about the calculated optimalalignment is provided by means of one of a plurality of methods (block116). In a first method, a rotation of the second image about the firstis initiated to alternate between a clockwise and counter-clockwiserotation, pausing at the optimal alignment point (block 117). Thismethod makes the image appear to “lock in” as it pauses at the midpointof the rotation.

In a second display method, the display is faded between the two images(block 118), pausing at the points at which only one of the images isdisplayed. During animation, the blood vessels may also be overlaid onthe images as an additional means of verification. This method impartsan apparent motion in the image if there is a misalignment, whilewell-aligned images have no apparent motion.

In a particular embodiment, the user is also permitted to perform amanual adjustment to the image alignment if the calculated optimalalignment is not deemed adequate (block 119). The user can choose toview the overlaid image or a dual display with the images separated butsize-matched so that any change to one display is reflected in that ofthe other (block 120). The user can then perform adjustments, forexample, by viewing or marking a common eye feature on each of theimages when the images are displayed separately and then combining theimages so that the marked feature can be aligned (block 121). Any numberof iterations can be carried out until the user is satisfied thatadequate alignment has been achieved. In an extreme case whereinadequate alignment is not achievable as determined by the user, it maybe decided that a new first image is needed, in which case “proceed”(block 122) may mean returning to a first imaging device for anadditional image.

In the foregoing description, certain terms have been used for brevity,clarity, and understanding, but no unnecessary limitations are to beimplied therefrom beyond the requirements of the prior art, because suchwords are used for description purposes herein and are intended to bebroadly construed. Moreover, the embodiments of the system and methodillustrated and described herein are by way of example, and the scope ofthe invention is not limited to the exact details of construction anduse.

Having now described the invention, the construction, the operation anduse of preferred embodiments thereof, and the advantageous new anduseful results obtained thereby, the new and useful constructions, andreasonable equivalents thereof obvious to those skilled in the art, areset forth in the appended claims.

1. A method for aligning a first and a second image of an eye comprising the steps of: determining a limbus location on a first eye image and a second eye image; aligning the limbus location of the first and the second eye images in two dimensions; determining a location of a second eye feature on the first and the second eye image; relatively rotating one of the first and the second eye image; performing a correlation on the first and the second eye image at a plurality of relative rotational positions using the second eye feature location; and from the correlation, identifying an optimal first and second image alignment.
 2. The method recited in claim 1, wherein the second eye feature comprises at least one of a blood vessel, a natural eye mark, and an artificial eye mark.
 3. The method recited in claim 1, wherein the second eye feature comprises a blood vessel, and the blood vessel location determining step comprises using a gradient filter.
 4. The method recited in claim 1, further comprising the step of displaying to a user the optimal first and second image alignment.
 5. The method recited in claim 4, wherein the first and the second image are stored as pixels, and wherein the displaying step comprises interleaving pixels of the first and the second image to form a composite image and displaying the composite image.
 6. The method recited in claim 4, further comprising the step, prior to the displaying step, of adjusting a relative contrast of the first and the second image to achieve a substantial match therebetween.
 7. The method recited in claim 6, further comprising the step, prior to the displaying step, of adjusting a relative brightness of the first and the second image to achieve a substantial match therebetween.
 8. The method recited in claim 4, wherein the displaying step comprises calculating and displaying an animation of a relative rotational motion of the first and the second image about the optimal alignment.
 9. The method recited in claim 4, wherein in the displaying step are included an image of the second eye feature in the first and the second image.
 10. The method recited in claim 4, further comprising, following the displaying step, the step of permitting the user to perform a manual adjustment of the optimal alignment of the first and the second image.
 11. The method recited in claim 10, further comprising the step of permitting the user to alternate between viewing a display of the first and the second image overlaid and a dual display of the first and the second image separated, the dual display having been size-matched.
 12. The method recited in claim 11, wherein the manual adjustment comprises viewing the dual display, marking at least one common eye feature on the first and the second image, viewing the overlaid display, and manually aligning the first and the second image using the marked common eye feature as reference.
 13. A system for aligning a first and a second image of an eye comprising: means for determining a limbus location on a first eye image and a second eye image; means for aligning the limbus location of the first and the second eye images in two dimensions; means for determining a location of a second eye feature on the first and the second eye image; means for relatively rotating one of the first and the second eye image; means for performing a correlation on the first and the second eye image at a plurality of relative rotational positions using the second eye feature location; and from the correlation, means for identifying an optimal first and second image alignment.
 14. The system recited in claim 13, wherein the second eye feature comprises at least one of a blood vessel, a natural eye mark, and an artificial eye mark.
 15. The system recited in claim 14, wherein the second eye feature comprises a blood vessel, and the blood vessel location determining step comprises using a gradient filter.
 16. The system recited in claim 13, further comprising means for displaying to a user the optimal first and second image alignment.
 17. The system recited in claim 13, wherein the first and the second image comprise electronic images stored as pixels, and wherein the displaying means comprises means for interleaving pixels of the first and the second image to form a composite image and for displaying the composite image.
 18. The system recited in claim 13, further comprising means for adjusting a relative contrast of the first and the second image to achieve a substantial match therebetween.
 19. The system recited in claim 18, further comprising means for adjusting a relative brightness of the first and the second image to achieve a substantial match therebetween.
 20. The system recited in claim 13, wherein the displaying means comprises means for calculating and displaying an animation of a relative rotational motion of the first and the second image about the optimal alignment.
 21. The system recited in claim 13, wherein the displaying means further comprises means for including an image of the second eye feature in the first and the second image.
 22. A system for displaying to a user an aligned first and second image of an eye comprising: a first imaging device for collecting a first image of an eye; a second imaging device for collecting a second image of the eye; a processor having resident thereon a software package comprising code segments adapted for: determining a limbus location on the first and the second eye image; aligning the limbus location of the first and the second eye images in two dimensions; determining a location of a second eye feature on the first and the second eye image; relatively rotating one of the first and the second eye image; performing a correlation on the first and the second eye image at a plurality of relative rotational positions using the second eye feature location; and from the correlation, identifying an optimal first and second image alignment; and a display device in signal communication with the processor adapted to display to a user the optimal first and second image alignment.
 23. The system recited in claim 22, wherein the second eye feature comprises at least one of a blood vessel, a natural eye mark, and an artificial eye mark.
 24. The system recited in claim 22, wherein the second eye feature comprises a blood vessel, and the blood vessel location determining step comprises using a gradient filter.
 25. The system recited in claim 22, wherein the first and the second image are stored as pixels, and wherein the software package further comprises code segments for interleaving pixels of the first and the second image to form a composite image, and for directing the display device to display the composite image.
 26. The system recited in claim 22, wherein the software package further comprises a code segment for adjusting a relative contrast of the first and the second image to achieve a substantial match therebetween.
 27. The system recited in claim 26, wherein the software package further comprises a code segment for adjusting a relative brightness of the first and the second image to achieve a substantial match therebetween.
 28. The system recited in claim 22, wherein the software package further comprises code segments for calculating an animation of a relative rotational motion of the first and the second image about the optimal alignment and for directing the display device to display the calculated animation.
 29. The system recited in claim 22, wherein optimal first and second image alignment display includes an image of the second eye feature in the first and the second image. 